A Level Chemistry OCR Practice Exam

What characterizes a permanent dipole within a molecule?

1. A uniform charge distribution

2. A small charge difference across a bond

3. A temporary shift in electron density

4. A complete charge separation

The correct answer is: A small charge difference across a bond

A permanent dipole in a molecule is characterized by a small but consistent charge difference across a bond. This occurs due to the unequal sharing of electrons between atoms with different electronegativities, resulting in a partial positive charge (δ+) on one atom and a partial negative charge (δ−) on another. This uneven distribution of electron density creates a dipole moment, which is a vector quantity pointing from the positive to the negative end of the dipole. The option indicating a uniform charge distribution would imply that there are no differences in electronegativity or electron density within the molecule, which would not produce a dipole. Similarly, a temporary shift in electron density refers to induced dipoles, which occur in nonpolar molecules under external influences, rather than a permanent dipole that exists under all conditions. Complete charge separation typically occurs in ionic compounds, where there is a full transfer of electrons resulting in fully charged ions, rather than the partial charges observed in polar covalent bonds. Thus, the choice highlighting the small charge difference across a bond accurately reflects the nature of a permanent dipole.